This invention relates to polymer blends of PVDF thermoplastics with FKM fluoroelastomers.
Rubber/plastic blends are known in the prior art. Prominent examples include the blends of nitrile/butadiene copolymer rubber (NBR) with polyvinylchloride (PVC), and blends of styrene/butadiene copolymer rubber (SBR) with so-called xe2x80x9chigh-styrene resinsxe2x80x9d (which are styrene/butadiene copolymers with typically 20% or less butadiene). In some cases, the plastic phase of a rubber/plastic blend may co-crosslink with the rubber phase, as in blends of SBR with high-styrene resins. Blends in which the plastic phase does not crosslink or graft with the elastomer phase are also useful, as in the case of NBR/PVC blends.
Rubber/plastic blends of FKM with xe2x80x9cTHVxe2x80x9d polymers from Dyneon are also known in the prior art. THV polymers are copolymers of tetrafluoroethylene (TFE), hexafluoropropene (HFP), and vinylidene fluoride (VDF), with VDF content well below 50% by weight. Such blends have been used in fuel-containment applications, such as fuel lines and fuel filler neck hoses.
FKM/PVDF and FKM/THV blends in which the FKM is present as crosslinked particles are also known in the prior art. This is believed to be the foundation of the xe2x80x9cFluoroprenexe2x80x9d product line from Freudenberg NOK. See, for example, the paper by Craig Chmielewski, xe2x80x9cFluoroprene: Freudenberg NOK""s new Fluorinated TPV,xe2x80x9d presented at the Performance Elastomers and TPEs 2001 seminar in Cleveland, Ohio May 14-15, 2001. Note that in such dynamically cured blends, the FKM phase is crosslinked and does not flow per se, which is quite different than the case of the present invention.
The invention is a moldable, extrudable, crosslinkable composition of matter involving at least two flowable, non-crosslinked polymer phases, wherein Phase 1 is composed primarily of an FKM fluoroelastomer and Phase 2 is composed primarily of a PVDF thermoplastic copolymer, plus crosslinking agents for the FKM fluoroelastomer. The invention also applies to the partially crosslinked articles that are derived from thermally crosslinking the above compositions. The FKM fluoroelastomer of Phase 1 is a copolymer of vinylidene fluoride, hexafluoropropene, and optionally also tetrafluoroethylene and/or various perfluorovinylethers, and/or various cure site monomers, which we shall refer to generically as xe2x80x9cFKM.xe2x80x9d Component 2 is a PVDF homopolymer or a copolymer of vinylidene fluoride with one or more co-monomers, including specifically hexafluoropropene (PVDF/HFP copolymers), tetrafluoroethylene (PVDF/TFE copolymers), and chlorotrifluoroethylene (PVDF/CTFE copolymers). The crosslinking agents used for the FKM may or may not also react with the PVDF.
The blends of this invention are shaped by molding, extrusion, or other methods of processing to final shape, usually but not necessarily below 140 degrees C. After shaping, the blends are thermally cured, usually but not necessarily at a temperature above 140 degrees C. In most cases, but not necessarily, the blends of this invention must be under pressure during curing to avoid blisters. The maximum practical cure temperature for the blends of this invention is often limited by the tendency of the cured parts to blister when the mold is opened (due to the sudden pressure decrease).
The FKM component of the blends of this invention can be crosslinked by any method known in the prior art, such as for example diamines or diamine-releasing chemicals, bisphenol cure systems, or peroxide cure systems. Cure systems that release minimal amounts of volatile compounds are preferred over cure systems that release a lot of volatile compounds, because volatile compounds lead to blistering.
Weight % fluorine in FKM is a critical variable that impacts especially resistance to swelling by hydrocarbons and permeation by fuels. Insofar as the FKM-rich Phase 1 forms the major part of the blends of this invention, it is vital to choose a high-fluorine content FKM if one wishes to obtain a high permeation resistance. Because of the importance of permeation resistance, a particular bisphenol-crosslinkable high fluorine-content FKM (Dai-El G-621, which contains 71.5% fluorine, approximately) has been used in most of the experiments performed in developing the present invention. The process of this invention is equally applicable to blends of PVDF and/or PVDF copolymers with other commercially significant grades of FKM, containing 65-73% combined fluorine, such as dipolymers of vinylidene fluoride with hexafluoropropene (e.g., Viton A from DuPont), low-temperature grades of FKM containing perfluorovinylether monomer residues, and/or various peroxide crosslinkable FKMs containing labile bromine or iodine, or vinyl groups.
The compositions of this invention may also contain additional components, such as fillers, fibers, pigments, and processing aides. A particularly useful group of examples of this invention comprise electrically conductive moldable, extrudable, crosslinkable compositions of matter in which an effective level of conductive carbon black and/or combinations of carbon black with larger particle size conductive fillers (such as graphite, silicon carbide, metal powders, or metal-coated mineral fillers) is mixed with the FKM/PVDF blend. Admixing of conductive carbon black into such a conductive blend may occur either simultaneously with formation of the blend, or the conductive carbon black may be pre-incorporated into the PVDF prior to mixing the PVDF with the FKM.
This invention features a moldable, extrudable, thermally crosslinkable composition of matter blend comprising about 50-99% by weight fluoropolymers, in which about 50-95% of the polymer content of the blend is an FKM fluoroelastomer and about 5-50% of the polymer content of the blend is one or more thermoplastic PVDF polymers or copolymers containing at least about 70% by weight vinylidene fluoride monomer units, and the crosslinked articles derived from processing and curing the subject composition of matter.
The composition may comprise one or more PVDF/HFP copolymers at a total level between 10-45% by weight of the polymer content of the blend. The composition may comprise a PVDF/CTFE copolymer at a level between 10-45% by weight of the polymer content of the blend. The composition may comprise a PVDF/HFP copolymer at a level between 10-44.5% by weight of the polymer content of the blend and a minor portion of a PVDF/CTFE copolymer at a level between 0.5-5% of the polymer content of the blend. The composition may comprise a PVDF/HFP copolymer at a level between 10-44.5% by weight of the polymer content of the blend and a minor portion of a THV copolymer with a melting temperature below 150xc2x0 C., at a level between 0.5-5% of the polymer content of the blend.
The composition may further comprise one or more platy fillers selected from the group of such fillers consisting of mica, talc, clay, and delaminated graphite, to accomplish a low-permeability composition. The platy filler may be composed at least in part of mica, talc, or clay which has been treated with an aminosilane.
The composition may comprise a PVDF/HFP copolymer at a level between 10-45% by weight of the polymer content of the blend, optionally a fluoroplastic processing aid at a level up to 5% by weight of the polymer, with the balance of the polymeric portion of the composition consisting of a high-fluorine FKM polymer, containing at least 71% by weight fluorine.
The composition may further comprise at least two conductive fillers of different size and shape, to accomplish electrical resistivity below 106 ohm-cm. The conductive fillers may comprise 2-4% by volume of a platy conductive filler selected from the group consisting of graphite powder, metal-coated mica, and metal flakes, plus 5-8% by volume of an electrically conductive carbon black which has at most 120 m2/gram BET surface area. The composition may further comprise a platy filler selected from the group consisting of mica, talc, clay, and delaminated graphite. The platy filler may be composed at least in part of mica, talc, or clay that has been treated with an aminosilane. The composition may further comprise one or more types of oligomeric poly-CTFE as a processing aid.
The FKM may be a peroxide-crosslinkable elastomer, and the composition may further comprise a free radical generating initiator and optionally a coagent. The FKM may be a peroxide-crosslinkable low-temperature FKM elastomer. The FKM may be a labile iodine-containing peroxide-crosslinkable FKM elastomer. The FKM may be a high-fluorine material with greater than 72% combined fluorine by weight. The composition may further comprise about 0.2-4 parts of a zinc salt of one or several fatty acids. The composition may comprise a PVDF/HFP copolymer at a level between 10-44.5% by weight of the polymer content of the blend, a conductive carbon black, and a minor portion of a THV copolymer with a melting temperature below 150xc2x0 C., at a level between 0.5-5% of the polymer content of the blend.